The present invention relates to an improved method and apparatus for removing oxygen from water, particularly injection water used in waterflooding oil reservoirs.
Fresh water as well as sea water generally contains between 7 and 14 ppm dissolved oxygen. When such water is used in waterflooding an oil reservoir the oxygen therein will cause corrosion of metallic parts exposed to high pressure and it may also promote bacteria growth clogging the pores of the reservoir. These are the main reasons why the oxygen content of such injection water is normally reduced to less than 0.05 ppm before the water is pumped to higher pressures. The oxygen removal generally takes place in at least two stages. The first stage could be a vacuum tower in which oxygen is stripped from water passing down through a packing material in countercurrent flow to a stripping gas which is introduced at the bottom of the tower and drawn off at the top thereof. The next stage of the process could be a chemical scavenging procedure such as treating with ammonium bisulphite to bind the oxygen. Descriptions of such apparatus are given inter alia by W. J. Frank (SPE Paper No. 4064, 1972) for a fresh-water installation at Seminole, Texas, and by R. W. Mitchell (J. Petr. Techn., June 1978, page 877-884) for seawater at the Forties field in The North Sea. The apparatus described by Frank uses 0.75 mol natural gas per m.sup.3 water as stripping gas. The Forties field uses a CO.sub.2 -containing stripping gas that seems to promote corrosion when the oxygen content drops below 0.1 ppm.
It is also possible to remove oxygen under vacuum without the use of a stripping gas, thus permitting a smaller diameter degassing tower and eliminating the need for drawing off and compressing to atmospheric pressure of a gas quantity much larger than the gas quantity liberated from the water. On the other hand, oxygen removal is less effective without the use of a stripping gas, necessitating a lower pressure and more vacuum stages in the degassing tower in order to reach the desired reduction of the oxygen content. This in turn, implies an undesirably high pressure ratio between the atmosphere and the lowest vacuum stage.
Removal of oxygen under vacuum without the use of a stripping gas is discussed for instance by J. McGill (Oil and Gas J., Oct. 1, 1973, p. 81-84) and by C. M. Hudgins and R. T. Hanson (Oil and Gas J., Feb. 15, 1971, p. 71-77). The latter publication is concerned with the installations at the San Miguelito field near Ventura, California, where seawater is used as injection water. The vacuum tower has a diameter of appr. 2.1 m and has three stages, each having 2.44 m of packing and a seal between the stages. Liberated gases and water vapour are drawn off from the top of each stage and compressed to atmospheric pressure. However, as also pointed out in the above McGill article, there are two rather important drawbacks associated with a such procedure. One is the above mentioned high pressure ratio requiring the gases to be compressed from the lowest stage up to atmospheric pressure. Thus, in order to obtain 0.1 ppm of oxygen from the last stage when deaerating water at 60.degree. F. (16.degree. C.) he contemplates a pressure ratio of 44. This places extra high demands on the compressors drawing gas from stages after the first stage and also requires a high energy consumption. The other drawback is uncertainty of degassing toward the bottom of the tower where the vapour rate goes to zero.